It is not easy to create a system that permits a base station to communicate individually with each of myriad radio tags in an area, particularly where the tags are required to be limited in cost and size and are required to function for a long time without a problem of a battery running down. Consider the particular case where the reading of the tags is intended to be an “area read”, meaning that the tags are distributed about an area and the base station is at some distance from many of the tags and the operator of the system does not have the luxury of being able to assume that a reader is moved to each of the tags seriatim for individual close-up reading.
Nearly all RF-ID tag systems in the prior art are systems that designed on the assumption that when a reader reads a tag, the reader and tag are close together or (in some systems) the reader has a high-gain antenna pointed directly at the tag and at no other tag. In this way, the tag is bathed in RF energy and responds specifically because it notices that it has been bathed in RF energy.
With such tags it is impossible to do an “area read”. Stated differently, if some antenna were positioned so as to be RF-linked with myriad tags, then bathing several or most of the tags in RF energy would simply yield an unintelligible burst of radio clutter. With such a system, the reader would receive confused and overlapping responses if the reader were juxtaposed or positioned so that more than one tag is in its reading path.
One design decision that can help with making “area reads” possible is the use of lower radio frequencies rather than higher frequencies. This is not easy to do but approaches such as those described in U.S. application Ser. No. 11/276,096 filed Feb. 14, 2006, incorporated herein by reference, can make it possible to use lower frequencies.
Another design decision that can help with making “area reads” possible is the use of tags that do not simply respond each time they are bathed in RF energy, but that instead respond only after certain conditions are satisfied, such as receipt of a message with particular content or development of a predefined internal state. Such tags are described in U.S. application Ser. No. 11/419,750 filed May 22, 2006, incorporated herein by reference. With such a system there may be a power/clock RF field that provides power and clock information to the myriad tags in the reading area, and then a different data RF field (typically at a higher frequency than the power/clock field) permits messages to be passed to the tags and permits tags to respond. Each tag typically has two distinct antennas, one for for receiving the power/clock and another for sending and receiving data messages. The base station may likewise have two distinct antennas, or may employ a single antenna directed to both purposes.
But even with these design decisions, a system designer cannot do as much as one would like to do in terms of area reads over substantial areas and in terms of large numbers of tags that are in varied orientations relative to the base station and that may have been “detuned” due to proximity to metal or liquids or other materials that distort electrical or magnetic fields. One would like to be able to pick up a particular tag to the exclusion of others. One would like to be able to pick up all or nearly all of the tags in an area, rather than only being able to pick up some of them.
It would thus be very desirable if additional design elements could be devised that would help to facilitate these aspects of performing “area reads”.